The goal of the proposed program is to develop a device for myocardial ablation using microwave heating and Passive Microwave Radiometry (PMR) for tissue temperature monitoring. The device consists of an applicator, a microprocessor-controlled solid-state microwave energy source at 915 MHz for heating and a PMR device at 4.0 GHz. Using PMR to monitor the temperature of a volume of tissue enables the device to deliver localized heating to tissue under feedback control. The applicator is a novel adaptation of an antenna formed with thin, flexible coaxial cable with a helically wound tip. This configuration is capable of simultaneous heating and temperature monitoring and allows intraoperative ablation or catheter ablation for treatment of cardiac arrhythmias, particularly ventricular tachyarrhythmias. The unique feature of this ablation method is its ability to create reproducible, uniform lesions with selectable depths from 2-3 mm up to several centimeters in less than a minute. Since PMR measures the temperature of an integrated volume of tissue, it is superior to ablation systems employing point source thermocouple measurements. At present, no other technique including DC shock, RF, laser or cryoablation generates such a broad, shallow lesion. PROPOSED COMMERCIAL APPLICATION: The proposed microwave heating method offers physicians a rapid, uniform and controlled technique for myocardial ablation to treat ventricular tachyarrhythmia in those patients who are poor candidates for antiarrhythmic drug treatment. The commercial market for such a device is large. Each year, approximately one million people in the United States die of cardiovascular disease. Of these, several hundred thousand die of sudden death, often the result of ventricular tachyarrhythmia related to myocardial infarction.